1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for touch sensing, and more specifically, a capacitive touch sensing device having a modulated back plate.
2. Description of the Related Art
Input devices including proximity sensor devices (also commonly called touchpads or touch sensor devices) are widely used in a variety of electronic systems. A proximity sensor device typically includes a sensing region, often demarked by a surface, in which the proximity sensor device determines the presence, location and/or motion of one or more input objects. Proximity sensor devices may be used to provide interfaces for the electronic system. For example, proximity sensor devices are often used as input devices for larger computing systems (such as opaque touchpads integrated in, or peripheral to, notebook or desktop computers). Proximity sensor devices are also often used in smaller computing systems (such as touch screens integrated in cellular phones).
Many proximity sensor devices utilize an array of sensor electrodes to measure a change in capacitance indicative of the presence of an input object, such as a finger or stylus, proximate the sensor electrode. Many schemes are possible for capacitive touch sensing. In one scheme, “matrix sensing,” sensor electrodes arranged in a grid or “matrix” are driven to generate a capacitive image. The sensor electrodes may be driven in an absolute capacitance mode, in which the sensor electrodes are driven with a signal to determine the degree of capacitive coupling between the sensor electrodes and an input object, if present.
Sensor electrodes driven in absolute sensing mode may experience effects related to parasitic capacitances between the sensor electrodes and conductive objects other than an input object. More specifically, conductive objects that are associated with the input device contribute to the capacitance sensed by a sensor electrode driven for capacitive sensing. The existence of parasitic capacitance reduces the ability to detect the presence of an input object. This issue is more acute in “in-cell” display embodiments, in which the sensing electrode is a part of a display pixel cell, and therefore the sensing electrode is very close to several conductive elements, such as the pixel electrode of a display cell, and the terminals of a pixel transistor, among others.
Thus, there is a need for an improved proximity sensor device.